1. Field of the Invention
The invention relates to a positive electrode current collector containing a conductive ceramic layer formed on the surface of a current collector substrate of titanium or a titanium alloy and a production method thereof.
2. Description of the Related Art
New materials in place of lead or lead alloy to be used for positive electrode current collectors for lead-acid batteries have been investigated. The materials are required to have high conductivity, insolubility in electrolyte solutions, electrochemical stability in electrolytic solutions, and high oxygen overvoltage. Conductive ceramics made of metal oxides, metal silicides, and the like have been known as those which satisfy these requirements. For example, there are SnO2, TixTa(1-x)O2, TiSi2, TiSi3, Ti5Si3, TaSi2, NbSi2, Nb5Si3, and Ti4O7.
However, positive electrode current collectors cannot be produced only from materials of these conductive ceramics. It is because the volume specific resistivity of such conductive ceramics is 10 Ω·cm or lower, however it is still too high as compared with those of common metals (10−6 to 10−5 Ω·cm or lower).
Therefore, it is proposed that conductive ceramic layers are formed on the surfaces of current collector substrates of titanium or titanium alloys to provide positive electrode current collectors of lead-acid batteries (e.g., Electrochemistry, Vol. 47 (668), the Electrochemical Society of Japan, 1979, and Electrochemistry, Vol. 48 (384), the Electrochemical Society of Japan, 1980). These positive electrode current collectors are obtained by coating conductive ceramics such as SnO2 (Sb-doped), PtOx, IrO2, and RuO2 on the surface of titanium and further coating β-PbO2 on the surface.
Further, also proposed are lead dioxide electrodes to be used for DSA electrodes for electrolysis which are produced by coating surfaces of current collector substrates of titanium or titanium alloy with conductive ceramics made of PtOx and further with α-PbO2 and β-PbO2 (e.g. reference to Japanese Patent Application Laid-Open (JP-A) No. 63-57791).
As described in these Documents, in the case where titanium provided with a conductive ceramic layer on the surface is used as a positive electrode current collector, since the conductivity of titanium is sufficiently high as compared with that of the conductive ceramic, the conductivity of the positive electrode current collector is ensured. Further, since titanium has high corrosion resistivity, it is basically hard to be dissolved in diluted sulfuric acid, which is an electrolyte solution. Moreover, if a conductive ceramic layer is formed, dissolution of titanium does not become a problem. Further, since titanium has a high melting point, titanium can stand high temperature around 500° C. in a step of forming the conductive ceramic layer. Titanium also has a characteristic that it is made passivated, however the passivation can be prevented by coating of titanium with the conductive ceramic layer and therefore, the passivation does not either become a problem. Further, since titanium is more economical than conductive ceramics, the material cost can be saved.
Because of the above-mentioned reasons, investigations on titanium or titanium alloys provided with conductive ceramics on the surface have been made.